This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast that independently powers two or more lamps under external control.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 Hz.)
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 40-65 kHz. Because the frequency of the inverter is much higher than 50-60 Hz., the inductors for an electronic ballast are much smaller than the inductor in a magnetic ballast.
Some ballasts include a boost circuit between the rectifier and the inverter. As used herein, a "boost" circuit is a circuit that increases the DC voltage, e.g. from approximately 170 volts (assuming a 120 volt input) to 300 volts or more, for operating a lamp and for providing power factor correction. "Power factor" is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e. indicating whether or not the voltage and current are in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one). The boost circuit, the inverter, or both, can be self-oscillating, triggered, or driven.
It is known in the art to control an electronic ballast with a microprocessor. U.S. Pat. No. 5,680,015 (Bernitz et al.) discloses a ballast for a high intensity discharge lamp wherein a microprocessor controls a driven half-bridge inverter having a series resonant, direct coupled output. U.S. Pat. No. 5,925,990 (Crouse et al.) discloses controlling a ballast for gas discharge lamps and for monitoring the operation of the lamps.
Despite the technology contained in an electronic ballast, the ballast is only part of a larger system including lamps and fixtures. In many installations, it is commonplace to have room lighting controlled by two switches. Typically, one switch operates one of three fluorescent lamps and the other switch operates the remaining two lamps. The intention is that the full light output is not always required and hence energy can be saved by having reduced light output during parts of the day or evening.
Frequently, the way to separately control lamps is by having two three-lamp fixtures operated by three two-lamp ballasts or else have one two-lamp ballast and one four-lamp ballast. Each ballast operates lamps in both fixtures. A frequent arrangement is to have the center lamps in each fixture powered by a two lamp ballast in one of the fixtures. The remaining lamps are either operated by one four-lamp ballast in one fixture or else by a two-lamp ballast in each fixture. These configurations are pre-assembled at the factory in the form of a master fixture, a satellite fixture and a "whip" or connecting conduit that extends between two fixtures. The fixtures and whip are shipped together as components which must be assembled in the manner intended. Further, the operating voltage for each assembly has to be specified in advance.
Shipping these related pieces and assembling them in the field is commonly described as a nightmare. As one can imagine from the number of possible combinations of components, confusion and mistakes are likely and cost the manufacturers and the contractors large sums of money.
The problem is compounded in localities that require power to be reduced to less than fifty percent of full power when one switch to a fixture is off. Simply switching lamps on or off does not comply because of power dissipated in the ballast and because the power actually dissipated in a lamp is not necessarily the same as the nominal power rating of the lamp.
It is known in the art to toggle the power to a ballast to provide dimming; e.g. U.S. Pat. No. 5,177,409 (Nilssen), U.S. Pat. No. 5,373,218 (Konopka et al.), and U.S. Pat. No. 5,798,620 (Wacyk et al.). This approach is not acceptable to contractors. What is needed is a single ballast that can be used in a fixture to control lamps in any desired combination, preferably at any operating voltage.
Even though electronic ballasts and other electronic devices are limited in the amount of electromagnetic energy that they can emit, a power line is not noise free. On the contrary, there is significant noise on the power lines, so much so that it is difficult to communicate reliably with a ballast. In particular, transient voltages or spikes, which are not infrequent, can cause a ballast to change mode of operation.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast that controls the total input power in accordance with a signal provided to the ballast.
Another object of the invention is to provide an electronic ballast that reduces total input power to fifty percent or less in response to a signal on a line input to the ballast.
A further object of the invention is to provide a electronic ballast that controls total input power to a prescribed amount by a combination of lamp outages and power to the remaining lamps.